1. Field of the Invention
The present invention relates to a driver circuit and a display device.
2. Description of Related Art
Along with recent developments in advanced image/information-oriented society, and popularization of multimedia systems, flat display panels such as a liquid crystal display device have gained increasing importance. The liquid crystal display devices have been widely used as a display device of a portable terminal device etc. because of its low power consumption, slimness, light weight, and other such advantages.
In general, the liquid crystal display device includes a liquid crystal display panel for displaying an image, and a driver circuit for driving the liquid crystal display panel. The liquid crystal display panel includes: a TFT array substrate on which pixel electrodes are arranged in matrix, and switching elements such as TFTs (thin film transistors) are connected with the pixel electrodes; a counter substrate having formed thereon a common electrode opposite to the pixel electrodes; and liquid crystal filled in between the two substrates, for example.
Up to now, the following method has been used as a method of driving a liquid crystal display panel. That is, the voltage applied to liquid crystal is changed to thereby change the orientation of liquid crystal grains, and change the transmittance for multi-gray-scale display. According to this method, the voltage is changed within a range from a threshold voltage at which the transmittance starts varying to a saturation voltage that does not induce any further change in transmittance, in accordance with a desired gray scale to thereby change the transmittance for multi-gray-scale display.
When the liquid crystal display device is driven with DC voltage, a problem arises in that the display image causes burn-in due to, for example, degradation of liquid crystal components, and contamination with impurities mixed in the liquid crystal display panel. Therefore, an AC driving system such as a dot inversion driving system for changing a polarity of the driving voltage from one pixel to another pixel has been generally used. In the case of using this AC driving system, the common electrode is alternately applied with the positive voltage and the negative voltage, which consumes much power. To that end, there has been proposed a technique of saving power consumption using a charge recovery circuit (see Japanese Patent Translation Publication No. 2001-515225, for instance).
FIG. 11 is a circuit diagram showing a driver circuit of a conventional liquid crystal display panel having a charge recovery circuit. As shown in FIG. 11, the liquid crystal display device 10 includes a liquid crystal display panel 11 for displaying an image and a driver circuit 12. The driver circuit 12 includes plural operational amplifiers 13 supplying display signals. Each operational amplifier 13 is connected with a source line DL in the liquid crystal display panel 11. Each source line DL is connected with a first or second switch. The first switches are connected with, for example, the odd-numbered source lines DL, and connect between the odd-numbered source lines and the odd-numbered charge recovery lines. The second switches are connected with, for example, the even-numbered source line DL, and connect between the even-numbered source lines and the even-numbered charge recovery lines.
The odd-numbered charge recovery line and the even-numbered charge recovery line are each connected with a straight switch and a cross switch. The straight switch connects between the odd-numbered charge recovery line and one electrode of a positive-charge capacitor 14, or between the even-numbered charge recovery line and one electrode of a negative-charge capacitor 15. The cross switch connects between the odd-numbered charge recovery line and one electrode of the negative-charge capacitor 15 or between the even-numbered charge recovery line and one electrode of the positive-charge capacitor 14. The other electrodes of the positive-charge capacitor 14 and negative-charge capacitor 15 are connected with the common electrode in the liquid crystal display panel 11. Further, a neutralizing switch connects between the even-numbered charge recovery line and the odd-numbered charge recovery line.
As regards the dot inversion display, the polarity of the supplied display signal is inverted between the adjacent source lines DL. Accordingly, during a driving period, the positive display signal is applied to a first line, a second line next to the first line is applied with the negative display signal, and a third line next to the second line is applied with the positive display signal. During a subsequent gate line driving period, the first line is driven with the negative voltage, the second line is driven with the positive voltage, and the third line is drive with the negative voltage.
It is assumed here that the odd-numbered operational amplifiers supply the display signals of the positive polarity relative to the reference voltage, and the even-numbered operational amplifiers supply the display signals of the negative polarity relative to the reference voltage. After the image display, the charge recovery is executed. Upon the charge recovery, the first and second switches are turned on. Thus, the even-numbered source lines DL are connected with the even-numbered charge recovery lines, and the odd-numbered source lines DL are connected with the odd-numbered charge recovery lines. Then, the straight switches are turned on. Through this operation, the odd-numbered charge recovery lines are connected with the positive-charge capacitor 14, and the even-numbered charge recovery lines are connected with the negative-charge capacitor 15.
Through the above operation, charges accumulated in the pixel electrodes are recovered to each capacitor. Thereafter, the even-numbered charge recovery lines and the odd-numbered charge recovery lines are disconnected from the positive-charge capacitor 14 and the negative-charge capacitor 15, respectively. Then, the neutralizing switch is turned on, thereby electrically connecting between the even-numbered charge recovery line and the odd-numbered charge recovery line to set the source line DL at a reference potential. After that, the neutralizing switch is turned off, and two cross switches are turned on. This establishes the connection between the even-numbered charge recovery lines and the positive-charge capacitor 14 and between the odd-numbered charge recovery lines and the negative-charge capacitor 15. As a result, charges accumulated in the capacitors are transferred to the pixel electrodes to save the power consumption.
In the case of using the above charge recovery circuit, the charges of the plural source lines DL should be recovered by use of the straight switches and the cross switches, each of which are connected with the even-numbered charge recovery lines and odd-numbered charge recovery lines in a one-to-one correspondence. Hence, it is necessary to use the straight switch and cross switch having a high withstand voltage. For integration of the driver circuit having such a charge recovery circuit, the circuit is manufactured through a high-voltage process.
In the high-voltage process, the larger gate length or gate oxide film thickness is required for increasing the withstand voltage of the switches. This leads to a problem of an increased chip size. Besides, the switches are applied with both the positive and negative driving voltages for the liquid crystal, so a power source voltage of the driver circuit needs to be twice or more as high as the driving voltage for the liquid crystal. As a result, the power consumption is increased.